I've been there twice during marathon weekend, and it didn't seem to impact the parks very much, if at all. Yes, I did see people walking around wearing their medals, but it wasn't tons of people.
The place that I did notice an impact was in the lobby and bus stop at our resort (Pop). There were extra things set up in the lobby like informational tables and signs describing the transportation to the expo and the race itself, but that was a tiny thing. At the bus stop, at certain times of the day there was a long line of people waiting for the bus over to the expo or the race start, but that didn't affect us because we were in a different line. I mean, I saw the stuff, but it didn't cause any kind of a problem for us.
Our bus did have to take a detour to get around part of where the race was being run (one of the times we were there the race was delayed several hours due to thunder and lightning), but if we hadn't been paying attention, we wouldn't have even noticed that.